Capacitor-current-feedback active damping has been widely adopted in digitally controlled LCL-type grid-connected inverters (GCIs). However, their digital control delay causes the system stability to deteriorate within a negative damping range. Particularly, in a weak grid, such GCIs are prone to be unstable against the grid impedance variation. To address the above problem, a delay compensation strategy is proposed for an LCL-type GCI, in which a second-order low-pass filter (LPF) is inserted

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... nto the capacitorcurrent-feedback path. This increases the virtual-positive-resistance-region from one-sixth of the sampling frequency to almost within the Nyquist frequency. Therefore, the robustness of the grid-connected inverter can be significantly enhanced against the grid impedance variation. Furthermore, to ensure the system stability and improve the injecting current tracking performance, a proportional complex integral (PCI) strategy is adopted. More specifically, a robustness analysis is conducted under discrete domain. Finally, a 2-kW prototype is constructed, and the experimental results confirm the effectiveness of the theoretical expectations. INDEX TERMS LCL-type grid-connected inverter, delay compensation, active damping, robustness, weak grid. NOMENCLATURE LCL Inductor-capacitor-inductor. GCIs Grid connected inverters. CCFAD Capacitor-current-feedback active damping. VPRR Virtual-positive-resistance-region. RHP Right-half-plane. PWM Pulse width modulaton. PCI Proportional complex integral. U dc Input DC voltage. S 1 − S 4 Switch tubes of the inverter. U in Output voltage of the inverter. L 1 , L 2 , C Inverter side / grid side filter inductor, filter capacitor. L g Grid impedance. U g Grid voltage. U PCC Voltage at the point of common coupling. SOGI Second-order generalized integrator. PLL Phase locked loop.